Reduction of noise



March 28, 1939.

I w. R. KOCH REDUCTION OF NO-ISE 4 Filed May 29, 1936 2 Sheets-Sheet li. g 3nventor Win ield H. Koch Qttomeu March 28, 1939.

W. R. KOCH REDUCTION OF N'OISE Filei May 29, 1936 2 Sheets-Sheet 2VVinfie'Zd 'R, Koch stwetltor M I Patented Mar. 28, 1939 PATENT OFFICEREDUCTION OF NOISE Winfield It. Koch, Merchantville, N. J., assignor toRadio Corporation of America, a corporation of Delaware Application May29, 1936, Serial No. 82,417

8 Claims.

This invention relates to the reduction of noise such as that producedin radio receivers and like apparatus by impulses extraneous to thesignal. It has for its principal object the provision of animprovedapparatus and method of operation whereby the efficiency of thesignal transmitting channel is varied or controlled to prevent thepassage of noise impulses having an amplitude substantially in excess ofthat of the signal.

Various types of noise reduction systems have been heretofore proposedor used. Some of these systems have involved the use of means forbalancing or neutralizing the effect of the impulses which tend toproduce noise. Others have involved the use of a remotely locatedantenna,

a limiter or peak clipper, special transmission circuits or means forshort circuiting, open circuiting or otherwise controlling thetransmitting eificiency of the .signal channel for the duration of theimpulses which tend to produce objectionable noise.

Some of these systems have not been altogether satisfactory for thereason that they are complicated and expensive or are not effective 5 toreduce the noise to a point where it is unobjectionable. In accordancewith. the present invention, these difi'iculties are obviated orminimized by the provision of an improved circuit arrangement and methodof operation whereby 39 relatively high level noise impulses are causedto impair the transmitting vefi'iciency of a radio receiver detector orare utilized to vary the load of the detector.

The invention will be better understood from 5 the following descriptionwhen considered in connection with the accompanying drawings, and

its scope is indicated bythe amended claims.

Referring to the drawings: Fig. 1 is a wiring diagram of a noisereduction 40 system including acontrol circuit provided witha pentodeelectron discharge device which operates in response to relatively highlevel noise impulses to reduce the control potential of a signal channeldiode detector and audio frequency Fig. 2 is a similar wiring diagramwhich differs from that of Fig. l in that grid control potential isapplied to the pentode tube through means including a voltage doublerand an im- 50 pedance device is provided in the detector cathode circuitfor more efiectively suppressing the noise,

Fig. 3 is a wiring diagram similar to that of Fig. 2, from which itdiffers in respect to the location of its ground terminal and to thepro- 55 vision of a diode detector amplifier grid resistor for keeping anegative potential on the grid, and

Figs. 4 and 5 are wiring diagrams which are respectively similar in somerespects to those of Figs. 1 and :2, but differ therefrom in that thenoise suppressing control potential is applied di- 5 rectly to a pentodedetector instead of to the detector through a pentode located in aseparate control channel.

The system of Fig. 1 includes an intermediate frequency amplifier IDfrom which signal im- 10 pulses are applied to the diode detectorelectrodes |||2 of a diode detector and audio amplifier l3. The highpotential side of the audio frequency output circuit is indicated at M,and plate potential for operating the various tubes of 15 the signal andcontrol channels is derived'from a resistor I5 connected between the. Bsupply terminals l6 and H. It Will be noted that the diode detector||-|2 is provided with a resistor 58 shunted by a capacitor l9, and witha resistor 20 20 connected in the circuit of its control grid 2|.

The control or noise suppression channel includes a secondary winding 22associated with the detector input transformer 23, a diode detector 24,a pentode tube 25 arranged to have its grid 25 potential controlled bythe output of the diode 24,

a resistor 26 connected in the plate circuit of the pentode 25, and adiode 2i interposed between the low voltage terminal of the resistor 26and the audio frequency amplifier grid 2|. 30

In the absence of noise impulses, signal impulses are detected by thediode ||2, amplified by the audio frequency amplifier l3 and suppliedbetween the terminal It and the ground terminal in the usual manner. Solong as the noise im- 5 vpulses are relatively low, the electrodes ofthe diode 27 are maintained at substantially the same voltage.Rectification is prevented as the impedance of the rectifier is high andno current flows. When the level of the noise impulses becomesrelatively high, however, the potential drop of the diode circuitresistor 28 increases, the pentode 25 is biased to cut-off, thepotential drop of the resistor 26 decreases, current is transmittedthrough the diode 21, and the grid 2| effectively shunted to cathode l2by the relatively low impedance of the path including diode 21 andresistor 26. Under this condition, practically all the voltage of thenoise impulse will be dissipated in resistor 20, which is of relativelyhigh resistance, and only a small part of the noise impulse voltage willbe impressed on the grid 2|. The level at which the suppressor actionbegins may obviously be controlled by changes in the fixed bias, plateor screen grid voltage of the pentode 25, by changes in the fixed biasof the diode 24, or by other suitable adjustments of the circuit.

The system of Fig. 2 differs from that of Fig. 1 in that (1) the diode24 is replaced by a voltage doubling rectifier 30 which is connectedthrough a capacitor 3! to the high voltage secondary terminal of theinput transformer 23; and (2) a resistor 32 shunted by a capacitor 33 isconnected in the cathode lead of the diode detector audio amplifier 13.As will be readily understood, the voltage doubling rectifier 30provides a more convenient and more effective control than the diode 24of Fig. 1, and the cathode lead resistor 32 is effective to control theaverage potential drop of the resistor I8, so that, for low percentagemodulation signals, the noise suppression will be more effective, thesurges being repressed in the downward as well as in the upwarddirection. The operation of the system of Fig. 2 will be readilyunderstood in view of the previous explanation in connection with Fig.l.

The system of Fig. 3 differs from that of Fig. 2 in that the pentode 25is replaced by a triode 40, and a resistor 4| is connected in the gridcircuit of the diode detector audio frequency amplifier I3 forpreventing a positive potential at the grid 2!. This system has theadvantage that the negative terminal of the B supply circuit isgrounded. Its operation is similar to that of the systems of Figs. 1 and2 and will be readily understood without detailed explanation.

The system of Figs. 4 and 5 are respectively similar to the systems ofFigs. 1 and 2, but differ therefrom in that (l) a pentode rectifier 50instead of a detector diode is utilized; (2) the output potential ofcontrol diode 24 is applied to the control grid 5| of the pentode; and(3) the B supply resistor I5 is connected through its high potentialterminal to a grid 52.

For large signal voltages the pentode 50 will be biased beyond cut-offand no detection can take place. The level at which the suppressionoccurs can be controlled by changing the return of the cathode of thediode, by a fixed bias on the control diode 24 or the like. The circuitsmay be simplified by using combination tubes. For example, the controldiode 24 in Fig. 4 may be part of a diode triode with the triode used asan. amplifier for audio voltages or as part of a diode pentode used forintermediate or audio frequency amplification.

Particular features may be incorporated in the receiver used with thesenoise suppression systems. For example, a separate channel amplified A.V. C. may be provided so that the characteristic curve of detectoroutput vs. signal input voltage will fall slightly with increasingapplied Signal strengths, and the level at the detector will be highestfor medium strength signals. Closer suppression will be then obtainedwith weak or medium strength signals, which is usually desirable.Various modifications of the circuits may be made to use test coils,shunt fed diodes, voltage doubling diodes, and the like.

I claim as my invention:

1. The combination of a signal channel including an electron dischargedevice provided with input and output circuits, means for controllingthe transmitting efficiency of said device in response to a detectedaudio frequency signal component, a control tube, a diode rectifierconnected control potential dependent on a signal component differentfrom that applied to the input circuit of said device.

2. The combination of a signal channel including an electron dischargedevice provided with input and output circuits, means for controllingthe transmitting efiiciency of said device in response to a detectedaudio frequency signal component, a control tube, a diode rectifierconnected between said input circuit and the output circuit of saidcontrol tube, and means including a voltage doubling rectifier forsubjecting said control tube to a control potential dependent on asignal component different from that applied to the input circuit ofsaid device.

3. The combination of a signal channel including an electron dischargedemodulator and amplifier device provided with input and outputcircuits, means for controlling the transmitting efiiciency of saiddevice in response to the audio frequency component of a demodulatedsignal, a control tube having a relatively low resistance outputcircuit, a. diode rectifier connected between said input circuit and theoutput circuit of said control tube, a relatively high resistance devicein circuit between the said input circuit and the diode rectifier, andmeans including a second rectifier for subjecting said control tube to acontrol potential dependent on a signal component different from thatsupplied to the input circuit of said device.

4. The combination in a signal transmission channel, of a signalamplifier device having a signal input circuit and a signal outputcircuit, a signal rectifier having an output circuit connected with saidinput circuit to apply thereto biasing and rectified signal potentialsresulting from signal rectification, a series resistor in saidconnection, a rectifier and a second resistor connected in series acrosssaid input circuit, said second resistor being of relatively lowresistance with respect to the first-named resistor, a second amplifierdevice having its space path connected through said second-namedresistor, said lastnamed amplifier device having a control grid, andmeans for subjecting said control grid to a potential .dependent on arectified component of the signal carrier wave.

5. In a signal amplifying channel, the combination of an amplifierdevice having a signal input circuit and a signal output circuit, meansfor rectifying a received modulated signal, said last-named means beingconnected to said input circuit for applying thereto the rectifiedcomponent of said modulated signal, a series resistor in saidconnection, a rectifier and a second resistor connected in series acrosssaid input circuit, said second resistor having a relatively lowresistance with respect to the first-named resistor, and means forapplying to said second resistor a potential dependent on a rectifiedcomponent of the signal carrier Wave.

6. In a signal amplifying channel, the combination of an amplifierdevice having a signal input circuit and a signal output circuit, meansfor rectifying a received modulated signal, said last-named means beingconnected to said input circuit for applying thereto the rectifiedcomponent of said modulated signal, a series resistor in saidconnection, a rectifier and a second rcsistor connected in series acrosssaid input circuit, said second resistor having a relatively lowresistance with respect to the first-named resistor, means for applyingto said second resistor a potential dependent on a rectified componentof the signal carrier wave, said last-named means including an amplifierdevice having a plate circuit including said second resistor, a dioderectifier device having an output circuit connected to said last-namedamplifier device to apply a controlling potential thereto, and means forapplying a received modulated signal to said lastnamed rectifier device.

7. The combination with an electron discharge amplifier device having, asignal input circuit and a signal output circuit, of a diode modulatedsignal rectifier having an output circuit connected with said inputcircuit to apply thereto the modulation and direct current components ofa rectified modulated signal, means providing a series impedance in saidconnection, a. diode rectifier device having a cathode connected withsaid input circuit at the high signal potential side thereof and havingan anode electrode connected with the low signal potential side of saidinput circuit, means providing an impedance in said last-namedconnection which is relatively low with respect to said first-namedimpedance means, and means for establishing a potential across saidlast-named impedance means which varies in accordance with a rectifiedcomponent of the signal carrier wave.

8. The combination with an electron discharge amplifier device asdefined by claim 7, further characterized by the fact that saidamplifier device is an audio frequency amplifier and that thefirst-named rectifier device is the second detector of a superheterodynereceiver.

WINFIELD R. KOCH.

